JPH08313667A - Reactor fuel assembly and nuclear fuel rod therefor - Google Patents

Abstract

PURPOSE: To absorb the irradiation growth of a nuclear fuel rod and prevent the penetration of foreign material accompanying coolant over the whole life in a reactor fuel assembly. CONSTITUTION: A reactor fuel assembly has an upper nozzle 11 and a lower nozzle 13 provided with a plurality of coolant channel holes and disposed to be opposed to each other, and a plurality of control rod guide tubes 15 whose both ends are connected to both nozzles respectively and extend in parallel to each other. An upper grid 11, an intermediate grid 19 and a lower grid 21 longitudinally spacedly fixed on the control rod guide tubes 15, a plurality of nuclear fuel rods 23 extending in parallel to each other between both the nozzles 11, 13 and supported by the grids 17, 19, 21 and a foreign material material filter grid 25 existing between the lower nozzle 13 and the lower grid 21 are provided, and the lower end part of the nuclear fuel rod 23 extending out in the foreign material filter grid 25 is formed of expandable structure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly used for combustion in a nuclear reactor, and more particularly to a structure of a nuclear fuel rod therefor.

[0002]

2. Description of the Related Art A fuel assembly used in a nuclear reactor, for example, a pressurized water nuclear reactor, generally has upper and lower nozzles which are provided with a plurality of coolant flow passage holes and are opposed to each other, and both upper and lower nozzles. A plurality of control rod guide tubes each having both ends connected to each other and extending in parallel to each other, generally about 10 supporting grids fixedly mounted on the control rod guide tubes at appropriate intervals in the longitudinal direction, both upper and lower sides. It is composed of a plurality of nuclear fuel rods extending parallel to each other between the nozzles and elastically supported by being inserted into the small supporting openings of the supporting grid.

The fuel assembly having the above structure is used by forming it in a reactor vessel by arranging it in a reactor vessel. However, there is a case where foreign matter is accompanied by the reactor coolant flowing therethrough, which is a fuel. If they enter the assembly and get caught between the fuel rods and remain there, malfunctions may occur. In order to prevent this, it has been proposed to provide a foreign matter filter grid between the lowermost lower support grid and the lower nozzle of the plurality of support grids. Its schematic structure is shown in FIG. In FIG. 7, the lower portion of the cladding tube 1 a of the nuclear fuel rod 1 is closed by a lower end plug 1 b, and the lower end portion of the nuclear fuel rod 1 is elastically supported by a lower support grid 3. A foreign matter filter grid 7 is provided on the upper side of the lower nozzle 5, and projections 7a and 7b called dimples are formed on the inside and outside thereof. The initial dimension of the interval between the upper surface of the lower nozzle 5 and the lower end of the nuclear fuel rod 1 is based on the irradiation growth amount of the nuclear fuel rod 1.

[0004]

In the above-mentioned structure, in the middle and late stages of the fuel life, the nuclear fuel rods 1 grow by irradiation and the lower end plugs 1b extend into the foreign matter filter grid 7 to prevent foreign matter from entering. Even if possible, it is difficult to rule out the existence of a gap between the lower end plug 1b and the foreign matter filter grid 7 as shown in the figure at the beginning of the fuel life, that is, immediately after the loading of the nuclear fuel assembly. Therefore, even if the foreign matter generation probability is very low at the beginning of the core life, if the foreign matter is generated and accompanied by the coolant, the foreign matter passes through the gap between the lower end plug 1b and the foreign matter filter grid 7. In some cases, the above-mentioned failure occurrence is not completely prevented.

In order to deal with the above problem, the length of the cladding tube is changed so that the lower end plug 1b of the nuclear fuel rod 1'is provided with the projections 7a and 7b of the foreign matter filter grid 7 as shown in FIG. It is also conceivable to place it in the enclosed position early in the fuel life. By doing so, it is convenient to completely prevent the passage of foreign matter at the beginning of the fuel life, but at the end of the fuel life, the irradiation-grown nuclear fuel rod 1'may interfere with the lower nozzle 5. , Actually hard to adopt.
In any case, interference between the nuclear fuel rod 1 ′ and the lower nozzle 5 due to irradiation growth must be avoided, and for this reason, the foreign matter filter grid becomes large, and the foreign matter filter grid also increases in size. Since it does not support ', there is a possibility that the lower end of the nuclear fuel rod 1'is not sufficiently supported. Further, since the irradiation growth amount of the nuclear fuel rod depends on the neutron irradiation amount thereof, if there is no margin in the interval between the lower fuel nozzle and the lower nozzle, depending on the position inside the loaded core of the fuel assembly or the position inside the fuel assembly, Interference may occur. Therefore, an object of the present invention is to provide a nuclear reactor fuel assembly and a nuclear fuel rod therefor capable of preventing the intrusion of foreign substances accompanied by a coolant without causing any trouble over the entire life of the fuel. .

[0006]

In order to achieve the above object, a reactor fuel assembly according to the present invention comprises upper and lower nozzles provided with a plurality of coolant passage holes and arranged to face each other,
A plurality of control rod guide tubes each having both ends connected to the same nozzle and extending in parallel with each other, an upper portion, an intermediate portion and a lower support grid fixedly mounted on the control rod guide tubes at intervals in the longitudinal direction. A plurality of nuclear fuel rods extending parallel to each other between the nozzles and supported by the support grid, and a foreign matter filter grid interposed between the lower nozzle and the lower support grid, wherein the foreign matter filter grid extends into the foreign matter filter grid. It is characterized in that a lower end portion of the ejected nuclear fuel rod is formed in a stretchable structure. Further, the nuclear fuel rod of the present invention has a cladding tube containing nuclear fuel pellets, an upper end plug and a lower end plug for closing the upper end and the lower end of the cladding pipe, a main body having a small diameter tip, and the small diameter tip. The lower end plug is composed of a large-diameter cap that is loosely fitted to the small-diameter cap, and a compression coil spring that is fitted to the small-diameter tip and elastically biases the large-diameter cap.

[0007]

In the reactor fuel assembly having the above structure, the lower end plug of the nuclear fuel rod is inserted into the foreign matter filter grid provided between the lower nozzle and the lower support grid to form the foreign matter capturing space. Therefore, the intrusion of foreign substances accompanying the coolant over the entire life of the fuel is prevented. When fuel combustion progresses and irradiation growth of nuclear fuel rods occurs, the tip of the lower end plug approaches the lower nozzle and finally contacts, but after that, the compression coil spring contracts and becomes larger than the main body of the lower end plug. Allows relative displacement with the diameter cap. In this way, the irradiation growth of nuclear fuel rods caused by the irradiation of neutrons is absorbed.

[0008]

Embodiments of the present invention will be described below with reference to the accompanying drawings. First, the overall structure of the fuel assembly 10 will be described. In FIG. 2, the upper nozzle 11 and the lower nozzle 13, which are substantially rectangular planes, are connected by a plurality of control rod guide tubes 15. Although not shown in FIG. 2, the upper nozzle 11
In the lower nozzle 13, a large number of coolant passage holes are distributed and formed. The control rod guide tube 15 includes an upper grid 17, seven intermediate grids 19 and a lower grid 21.
Are fixed at intervals. Each grid 17,1
Reference numerals 9 and 21 each have an egg frame shape, and the number of intermediate grids 19 varies depending on the size of the fuel assembly 10. Of the many small openings in each grid 17, 19, 21 that do not include the control rod guide tube 15, the nuclear fuel rod 2
3 are inserted and supported one by one. Furthermore, the lower nozzle 1
A foreign matter filter grid 25 is fixed to the control rod guide tube 15 between the lower grid 3 and the lower grid 21.

FIG. 1 shows a lower nozzle 13 and a lower grid 2.
1, the mutual relationship of the foreign matter filter grid 25 and the nuclear fuel rods 23 at an early stage of the fuel life is shown in an enlarged manner. In FIG. 2, the lower end opening of the cladding tube 27 surrounding the stack of fuel pellets (not shown) is closed by a lower end plug 30. The lower end plug 30 has a main body 3 having a small tip.
1, a large-diameter cap 33 loosely fitted to the tip small-diameter portion, and a compression coil spring 35 fitted to the small-diameter portion. An axial distance of distance A is provided between the main body 31 and the large-diameter cap 33. Has been formed.

Next, the structure of the foreign matter filter grid 25 will be described with reference to FIGS. 3 and 4. As is particularly clear from FIG. 4, the foreign matter filter grid 25 is formed by intersecting and combining thin strips at a right angle, and projections or upper dimples 25a are formed on the upper side of the strip and lower dimples 2 are formed on the lower side.
5b are formed by integral molding. The upper and lower dimples 25a and 25b project on the opposite surface of the strip plate. FIG. 3 is a top view of the foreign matter filter grid 25, which shows almost one quadrant thereof.
The above-mentioned dimples are not formed in the portion where the control rod guide tube enters instead of the nuclear fuel rods 23.

In the fuel assembly 10 having the above-described structure, the relationship between the lower nozzle 13 and the nuclear fuel rods 23 is shown in FIG. 1 at the beginning of the fuel life immediately after being loaded in the core in the reactor vessel (not shown). As shown, the coolant that has flowed through the flow path hole 13 a of the lower nozzle 13 flows between the foreign matter filter grid 25 and the large diameter cap 33, and further flows between the lower grid 21 and the covering pipe 27. Absorbs the nuclear reaction heat. Therefore, if the coolant is accompanied by foreign matter, it is trapped between the large-diameter cap 33 and the upper and lower dimples 25a and 25b of the foreign matter filter grid 25 to prevent further intrusion.

As the combustion progresses, mainly the cladding tube 27 of the nuclear fuel rod 23 is exposed to neutrons to grow by irradiation, and the state shown in FIG. 5 is obtained. That is, the large-diameter cap 33 is in contact with the lower nozzle 13, but the compression coil spring 35 remains at the initial length. When combustion further progresses and the end of life is reached, the state shown in FIG. 6 is reached. In this state, the irradiation growth of the nuclear fuel rods 23 is further advanced, but the compression coil spring 35 contracts to allow relative displacement of the large diameter cap 33, and the tip of the main body 31 and the inner end surface of the large diameter cap 33 come into contact with each other. There is. However, no axial load other than spring load is applied to the nuclear fuel rod 21. Nuclear fuel rods 21 in the middle and end of fuel life
The behavior of the lower end plug 30 of FIG. 1 is as shown and described in FIGS. 5 and 6, but the horizontal relative relationship between the large-diameter cap 33 and the foreign matter filter grid 25 does not change so much. As I explained, the coolant flows,
If there is a foreign material in the wake, it is also captured.

[0013]

As described above, according to the invention of claim 1, since the lower end plug of the nuclear fuel rod has an expandable structure, the irradiation growth of the nuclear fuel rod is absorbed, and the lower end plug and the foreign matter are absorbed. The relationship with the filter grid can be maintained in a predetermined good state over the entire life of the fuel, and the wake foreign matter of the coolant can be reliably captured, and the entry into the fuel assembly can be prevented.
Further, according to the second aspect of the present invention, the irradiation growth of the cladding tube portion of the nuclear fuel rod is absorbed by the elastic telescopic structure of the main body of the lower end plug and the large-diameter cap loosely fitted in the main body of the lower end plug. No axial load is generated and reliability can be maintained.

[Brief description of drawings]

FIG. 1 is an enlarged partial sectional view showing a main part of an embodiment of the present invention.

FIG. 2 is an elevational view showing a premise structure of the embodiment.

FIG. 3 is a partial plan view of the embodiment.

FIG. 4 is a partially enlarged perspective view showing a part of the embodiment.

FIG. 5 is an explanatory view of the operation of the above embodiment.

FIG. 6 is an explanatory view of the operation of the above embodiment.

FIG. 7 is a partial cross-sectional view showing an example of a conventional structure.

FIG. 8 is a partial cross-sectional view showing an example of a conventional structure.

[Explanation of symbols]

 10 Fuel Assembly 11 Upper Nozzle 13 Lower Nozzle 15 Control Rod Guide Tube 17 Upper Grid 19 Intermediate Grid 21 Lower Grid 23 Nuclear Fuel Rod 25 Foreign Particle Filter Grid 30 Lower End Plug 31 Main Body 33 Large Cap 35 Compression Coil Spring

Claims (2)

[Claims] 1. An upper and lower nozzle provided with a plurality of coolant flow passage holes and arranged to face each other, a plurality of control rod guide tubes having both ends connected to the both nozzles and extending in parallel to each other, and the same control. An upper portion, an intermediate portion and a lower supporting grid fixed to the rod guide tube at intervals in the longitudinal direction, a plurality of nuclear fuel rods extending in parallel between the nozzles and supported by the supporting grid, the lower nozzle and the In a nuclear reactor fuel assembly having a foreign matter filter grid interposed between a lower support grid and the lower support grid, the lower end portion of the nuclear fuel rod extending into the foreign matter filter grid is formed to have an expandable structure. Reactor fuel assembly. 2. A body having a cladding tube containing nuclear fuel pellets, an upper end plug and a lower end plug for closing the upper end and the lower end of the cladding pipe, a main body having a small diameter tip, and a loose fitting to the small diameter tip. A nuclear fuel rod for a nuclear fuel assembly, wherein the lower end plug is composed of a large-diameter cap and a compression coil spring fitted to the small-diameter tip and elastically biasing the large-diameter cap.